The invention relates to a process for monitoring the refrigerant fill-level in a refrigeration system and, in particular, to a refrigeration system in a motor vehicle. The refrigeration system includes a compressor which divides a refrigerant circuit into a high-pressure side and a low-pressure side.
A process of this type is known from German Patent document DE 41 24 363 A1. The fill-level is herein monitored on the principle of supercooling calculation. From the measured pressure value, the vapor-pressure temperature is determined a vapor-pressure curve of the refrigerant which is used. By obtaining the difference between the vapor-pressure temperature and the measured refrigerant temperature, the supercooling value is calculated. It was herein recognized that this supercooling value is a measure of the quantity of refrigerant present.
It is important in this procedure to bear in mind that the supercooling value, apart from the quantity of refrigerant present, is additionally dependent upon the outside temperature, upon the efficiency and ventilation, i.e. cooling, of a condenser located within the refrigeration system, upon the compressor rotation speed, and upon the type of compressor, i.e., a regulated or unregulated compressor. The known method definitely provides a reliable indication of the refrigerant fill-level for refrigeration systems having an unregulated compressor, since in this case all of the influencing variables are able to be sufficiently accurately recorded. Unregulated compressors of this type, which run either at full capacity or are cut out, operate on a synchronized basis. Where such a compressor is running and the system is in a steady state, indicative supercooling values are able to be determined provided that the condenser is evenly ventilated, i.e. where the travelling speed is constant. The outside temperature and the compressor rotation speed here have only a small influence upon the determined supercooling value, whereas contaminations of the refrigerant, e.g. an excessive proportion of foreign gas or moisture, have a large effect. This can be taken into account by using a correspondingly suitable vapor-pressure curve when calculating the supercooling. By measuring each or some of the variables for refrigeration systems having an unregulated compressor, such as the outside temperature, travelling speed, compressor operating state, operating state of any condenser booster fan and evaporator temperature, time domains can consequently be determined. In the time domains, the computed supercooling value can be used as a reliable measure of the refrigerant fill-level. The size of any remaining disturbance influences likewise is determinable and is able to be taken correspondingly into account when analyzing the supercooling.
In the known process, in order to monitor the fill-level, it is consequently envisioned to determine the supercooling value at time intervals until such time as a quasi-stable state for this value is recognized. Each of, or a part of, the supercooling values for the analysis are used without any reciprocal comparison of the supercooling values for fill-level control purposes.
Where a regulated compressor is used, the delivered quantity of refrigerant can be altered in multiple steps or by a continuous adjustment. The known calculation of the supercooling value provides, in this case, no direct pointer to the refrigerant fill-volume. Although there is the possibility of keeping such a compressor forcibly switched off for a sufficiently long time period, so that, when switched back on, it will certainly operate at full capacity, thereby providing reliable delivery-rate information for this operating phase, this procedure has a comfort-reducing effect because of the necessary compressor down-time.
There is therefore needed a process of the above-mentioned type by which the refrigerant fill-level can be monitored comfortably and reliably, by determining the supercooling value, even for refrigeration systems having a regulated compressor.
These needs are met according to the present invention by a process for monitoring the refrigerant fill-level in a motor vehicle refrigeration system having a compressor which divides a refrigerant circuit into a high-pressure side and a low pressure side. At time intervals, the relative pressure and temperature are measured on the high-pressure side. Using the measured pressure, on the basis of a refrigerant-specific function, an assigned temperature is determined. By subtracting the measured from the assigned temperature, a refrigerant-supercooling value is determined. This value can be analyzed as a measure of the refrigerant fill-level. At least during predefinable operating phases of the refrigeration system a maximum value is used for the evaluation of the refrigerant fill-level. The maximum value is set at the start of this operating phase to the last determined supercooling value and in the course of this operating phase to a respectively newly determined supercooling value which is greater than the maximum value obtained at the corresponding instant.
Underlying the present invention is the recognition that the supercooling value constantly declines and does not increase, on the one hand, as the quantity of refrigerant falls, but also, on the other hand, when disturbance variables encroach into the system and/or the regulated compressor delivers a quantity of refrigerant which is less than the maximum possible. Consequently, a supercooling value which is indicative of the fill-level is always obtained whenever the regulated compressor delivers at full capacity and no disturbance variables encroach. It is not however necessary to define this instant. For, if otherwise predetermined operating conditions are obtained, the influence of which is known, then the delivery of a smaller quantity of refrigerant by the regulated compressor and/or the encroachment of disturbance variables into the system during an operating phase of this type results, at most, in a drop in the supercooling values which are continually calculated during this operating phase. For this reason, a reliable fill-level indication in such an operating phase is made possible by the fact that a respective maximum value is used for evaluating the fill-level from the calculated supercooling values since this value is representative of the fill-level.
A further embodiment of the invention for a motor vehicle air-conditioning system has the advantage that, when the vehicle is stationary, the supercooling value which is instantaneously determined is simultaneously used immediately, in each case, as a measure of the refrigerant fill-level. This is desirable, in particular, for servicing works. This normally constitutes a usable fill-level indication, even without the creation of a maximum value. This is because when the vehicle is stationary, i.e. the travelling speed is equal to zero, the condenser of the air-conditioning system is only slightly ventilated and the efficiency of the system is consequently poorer, for which reason the regulated compressor, is highly likely to operate at full capacity.
A further embodiment of the invention has the advantage that even a gradual loss of refrigerant and the, where appropriate, resulting drop in fill-level below a predefined threshold value, is thereby discernable. For if a maximum value, which has been forcibly successively decremented in this manner, is used for monitoring the fill-level, then, if the fill-level remains constant, this maximum value is automatically increased again from time to time by a recorded higher supercooling value. If, in the further course of operation, no such automatic increase comes about, then this is consequently an indication that the fill-level has diminished during this operating phase.
Preferably, an underfill warning signal and/or a cutting-out of the refrigeration system is envisioned where the maximum value, used to evaluate the refrigerant fill-level, for a threshold time period which in each case is suitably pre-chosen remains less than a respectively suitably pre-chosen threshold value. The threshold time periods and threshold values for the warning signal and cut-out can herein be chosen to be equal or varied in size, a favorable realization consisting in providing equal-sized threshold time periods and, for the warning signal, a larger threshold value than for the cut-out. This is so that the warning signal is definitely effected prior to the cut-out of the refrigeration system. Expediently, the possibility is provided of halting the progression of the respective threshold time period for as long as the compressor is switched off or the vehicle is stationary, and to reset it whenever the vehicle is parked or the maximum value again exceeds the corresponding threshold value so as then to be able to start afresh the progression of the threshold time period.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.